Many exhaust lines are now equipped with NOx reducing devices of the SCR (Selective Catalytic Reduction) type. These devices are provided to convert NOx into gaseous N2. The reducing agent used is typically gaseous ammonia.
The injection device according to the invention is provided to be installed on such exhaust lines. The liquid reagent is then either a liquid ammonia solution, or a solution containing an ammonia precursor, such as urea.
When the exhaust gases are at a temperature below approximately 180° C., the transformation of the urea into NH3 is very difficult to do, and leads to deposits that may obstruct the exhaust line over time. These deposits may cause NH3 emission peaks during the temperature increase of the exhaust gases, if that increase leads to resorption of the deposits.
However, the SCR catalyst begins to be effective at lower temperatures, approximately 120° C. It also has an NH3 storage capacity, which makes it possible to manage the transitional phases where the demand for NH3 increases abruptly. The impossibility of injecting ammonia or urea solution at low temperatures makes it impossible to take advantage of that storage capacity. Thus, in the case of operation with a high load following a period with no possible injection, it is temporarily not possible to reduce the NOx.
In the future, approval cycles for exhaust line pollution control will incorporate colder and more realistic operating conditions for traveling at low temperatures, in particular in an urban cycle.
Furthermore, it is desirable to broaden the usage conditions for the NOx reducing system, and to that end it must be possible to inject liquid reagent containing the reducing agent, or the precursor for said reducing agent, at lower temperatures.
This means heating the injection zone of the liquid reagent to accelerate the evaporation of the reducing agent and the conversion of the precursor of said reducing agent if necessary.
Several technical solutions may be considered to heat the injection zone.
A first possibility consists of offsetting the combustion of the fuel, by performing a post-injection to heat the exhaust gases leaving the engine. This first solution causes excess fuel consumption, and potentially deterioration of the motor oil by diluting unburned products.
A second solution consists of electrically heating the exhaust gases. However, such heating requires significant electricity, which is detrimental to the fuel consumption of the vehicle. Furthermore, it is very difficult to install the electrical heating members due to the very small packaging space available to install the reducing agent injection device onboard the vehicle.
The best solution consists of heating only the impactor(s). These impactors are designed to receive the jet of liquid reagent, and to break it down into finer droplets, which favors their vaporization.
In fact, the majority of the jet of liquid reagent is sprayed on a relatively limited surface, namely the impactors, and the heating of that surface favors the vaporization and production of gaseous ammonia, even at a low temperature of the exhaust gases. The surface to be heated is relatively small, such that the electricity consumption and the size of the heating members are limited.
The heating device must make it possible to bring the impactor(s) to a high temperature, quickly. It must also be able to withstand the thermal and chemical stresses related to installation inside an exhaust line. The heating device must in particular be able to withstand exhaust gas temperatures of up to 700° C., and exposure to aggressive chemical elements like those resulting from the transformation of the precursor for the reducing agent, for example the isocyanic acid coming from the decomposition of the urea.
U.S. Pat. No. 6,969,492 describes one example of heating the impactors to favor evaporation of the liquid reagent. However, this document does not precisely describe how to heat the impactor(s) in practice.